Currently, it is done once per slice-thread, leading to
one warning per slice-thread in case a YUVJ pixel format
has been originally used.
This also fixes the anomaly that said parameter are only
updated for the user-facing context (whose values are retrievable
via av_opt_get()) if slice-threading is not in use.
Fixes ticket #9860.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>